1. Field
The present invention relates to a RAID apparatus, a controller of the RAID apparatus and a write-back control method of the RAID apparatus.
2. Description of the Related Art
Hard disk drives characterized by data non-volatility and high capacity are in widespread use as external recording devices of computers. A Redundant Arrays of Independent (Inexpensive) Disks (RAID) apparatus is used for duplicating data and storing the data in a plurality of hard disk drives, or storing the data with redundant information such as parity data.
A RAID controller apparatus controlling data transmission between the RAID apparatus and a host computer may be equipped with a high capacity cache memory. The high capacity cache memory may buffer data to improve a data rate. The RAID controller connects to a plurality of hard disk drives and controls them in accordance with each RAID level.
With a RAID apparatus, when the host computer writes data, the data are stored in the cache memory and then written back to the hard disk drives asynchronously. The data write-back to the hard disk drives is executed from the least-frequently accessed data with the Least Recently Used (LRU) method in consideration of overall performance of the RAID apparatus.
For a typical RAID apparatus, a summation of the capacities of the hard disk drives is much higher than a capacity of the cache memory. Therefore, efficient write-back is required to increase free space of the cache memory to use the RAID apparatus in an effective way.
However, a load on the hard disk drive will be heavy and the write-back efficiency of the entire RAID apparatus will be reduced if the write-backs are concentrated on a specific hard disk drive. Hence, the hard disk drive that is frequently used for the write-back is exempt from a write-back scheduling.
A RAID apparatus needs a buffer to calculate the parity in the write-back when supporting a RAID4, RAID5 or RAID6 level. Consequently, the more hard disk drives for RAID4, RAID5 or RAID6 levels are used, the more buffers are needed. Needing more buffers causes a problem in that even when the loads on the hard disk drives are light, the write-back process may be suspended due to a buffer shortage.
Other accesses to the cache memory are exclusively controlled during the writing-back process. Thus, when the write-back process is suspended, other processes commanded by the host computer are interfered with for a certain amount of time. In addition, when the write-back being processed is revoked due to RAS processing, the withdrawal takes time.